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Thromb Haemost
DOI: 10.1055/a-2784-0699
Invited Editorial Focus

Platelet-Leukocyte Aggregates: Targeting the Crosstalk

Authors

  • Daniel I. Simon 

    1   University Hospitals Harrington Heart & Vascular Institute and the Blood, Heart, Lung & Immunology Research Center, Case Western Reserve University School of Medicine, Cleveland, OH, United States

  • Edward F. Plow

    2   Department of Heart, Blood, and Kidney Research, Cleveland Clinic Foundation, Cleveland, OH, United States

Funding Information This work was supported by a National Institutes of Health grant to DIS and EFP (P01 HL154811).
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Heterotypic cell–cell interactions between vascular and hematologic cells (e.g., endothelial cells–leukocytes and dendritic cells–T cells) play an important role in innate and adaptive immunity, inflammation, and thrombosis. Cell adhesion molecules mediate these interactions. Recruitment of circulating leukocytes to vascular endothelium requires multi-step adhesive and signaling events, including selectin-mediated attachment and rolling, leukocyte activation, as well as integrin-mediated firm adhesion and diapedesis that result in the infiltration of inflammatory cells into the blood vessel wall.[1] Firm attachment is mediated by members of the β1-integrin (VLA-4, α4β1; α9β1) and β2-integrin (LFA-1, αLβ2,; Mac-1, αMβ2; p150,95, αXβ2; and αDβ2) families, which bind to endothelial counter ligands (e.g., vascular cell adhesion molecule-1, VCAM-1; intercellular adhesion molecule-1, ICAM-1), endothelial-associated extracellular matrix proteins (e.g., fibrinogen, fibronectin), or glycosaminoglycans.

Leukocyte recruitment and infiltration also occur at sites of vascular injury where the lining endothelial cells have been denuded, and platelets and fibrin have been deposited. A similar sequential adhesion model of leukocyte attachment to and transmigration across surface-adherent platelets has been proposed.[2] The initial tethering and rolling of leukocytes on platelet P-selectin[3] are followed by their firm adhesion and transplatelet migration, processes that are dependent on Mac-1[2] and multiple platelet counter-receptors, including GPIbα, JAM-3, ICAM-2, and fibrinogen bound to GPIIb/IIIa ([Fig. 1]). Leveraging Mac-1-deficient leukocytes, platelets from Bernard Soullier Syndrome patient with genetic deficiency of GPIbα, and antibodies that specifically block Mac-1 binding to GPIbα, but not other Mac-1 ligands, we have shown that stable platelet-leukocyte aggregates (PLAs) require platelet GPIbα:leukocyte Mac-1 binding.[4] [5] Importantly, disrupting platelet GPIbα:leukocyte Mac-1 broadly regulates the biological response to blood vessel/tissue injury in models of restenosis,[6] vasculitis,[7] glomerulonephritis,[8] experimental autoimmune encephalomyelitis/multiple sclerosis,[9] and arterial thrombosis.[10]

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Fig. 1 (A) Sequential adhesion model of leukocyte attachment to and transmigration across surface-adherent platelets. Initial tethering and rolling of leukocytes on platelet P-selectin are followed by their firm adhesion and transplatelet migration, processes that are dependent on Mac-1 and multiple platelet counter-receptors, including GPIbα and fibrinogen bound to GPIIb/IIIa. (B) The formation of platelet-monocyte aggregates induces bidirectional “outside-in” signaling via platelet GPIba:monocyte Mac-1 binding that amplifies pro-inflammatory and pro-thrombotic cellular responses. Source: Original illustrations by: ©2025 Virginia Ferrante-Iqbal.

The formation of PLAs, and specifically platelet-neutrophil aggregates (PNAs) and platelet monocyte aggregates (PMAs), induces bidirectional “outside-in” signaling that amplifies pro-inflammatory and pro-thrombotic cellular responses ([Fig. 1]). Ligand engagement of Mac-1 generates “outside-in” signals leading to inflammatory cell activation and induction of genes encoding for IL-1β, TNF-α, and tissue factor.[11] [12] Importantly, we reported previously that Mac-1 associates with IL-1 receptor-associated kinase and promotes activation of NFκB[13] and also orchestrates pro-inflammatory M1 differentiation by downregulating the forkhead transcriptional factor FOXP1.[14] [15] We have also shown that platelet GPIbα engagement of Mac-1 leads to phosphorylation of Akt and activation of GPIIb/IIIa.[16] Inflammatory mediators released by platelets as a consequence of this platelet-leukocyte “crosstalk” include, among others, platelet-derived growth factor, platelet factor 4, CD154/CD40 ligand, RANTES, thrombospondin, transforming growth factor-β, nitric oxide, and S100A8/A9. Therefore, in addition to promoting accumulation of leukocytes at sites of platelet coverage within the vasculature, binding of platelets to neutrophils and monocytes influences key cellular effector responses by inducing leukocyte and platelet activation, augmenting cell-adhesion molecule expression, and generating signals that promote integrin activation, chemokine synthesis, and respiratory burst.

In this issue of “Thrombosis and Haemostasis,” Peshkova et al[17] determined the levels, composition, and cellular reactivity of PLAs in patients with COVID-19, rheumatoid arthritis (RA), or systemic lupus erythematosus (SLE) compared to healthy controls. PLA levels were significantly higher in all patient groups compared to controls, with PNAs predominating in the COVID-19 patients and PMAs in the blood of RA and SLE patients. One of the most impressive aspects of this study was the author's ability to exploit flow cytometry to also demonstrate “outside-in” signaling in PLAs compared to nonaggregate constituents (platelets, neutrophils, or monocytes without platelet CD41+ staining) by showing that P-selectin expression and activated GPIIb/IIIa are increased in platelets within PNAs and PMAs compared to platelet gate alone. Similarly, phospho-protein kinase C was increased in monocytes within PMAs compared to monocytes alone. This study highlights distinct changes in the number and composition of PLAs in inflammatory diseases of various etiologies associated with the altered functionality of innate immune cells.

These flow cytometric assays could have important implications for future clinical trials targeting PLAs, which are elevated in additional diverse conditions including, among others, acute myocardial infarction/acute coronary syndromes, hypertension, pulmonary arterial hypertension, chronic thromboembolic pulmonary hypertension, diabetes, inflammatory bowel disease, sickle cell disease, sepsis, Kawasaki disease, nephrotic syndrome, acute lung injury/adult respiratory distress syndrome, and chronic liver disease. Crizanlizumab, a humanized anti-P-selectin antibody targeting P-selectin:PSGL-1-mediated PLAs and leukocyte-endothelial interactions, received FDA approval in 2019 for the prevention of sickle cell vaso-occlusive crises based on the randomized, placebo-controlled SUSTAIN trial.[18] The finding of elevated PLAs, and particularly PNAs, in acute COVID-19 infection may also have important implications for potential new therapeutic approaches for postacute sequalae of SARS-CoV-2 infection (PASC), which is associated with chronic inflammation and elevated thrombotic risk and increased hazard ratios for myocardial infarction, stroke and all-cause mortality (HR: 2.09)[19] and deep vein thrombosis (HR: 2.09) and pulmonary embolism (HR: 2.93)[20] within 12 months. The ACTIV-4a Platform Trial randomized hospitalized patients with moderate or severe COVID-19 to a single infusion of crizanlizumab targeting P-selectin:PSGL-1 plus standard of care versus standard of care alone. Crizanlizumab did not improve organ-support-free days (alive without cardiovascular or respiratory support through day 21) in patients hospitalized with COVID-19.[21] There are no approved therapies for PASC. Potential receptor:counter-receptor targets in addition to P-selectin:PSGL-1 include platelet GPIbα:leukocyte Mac-1 and S100A8/A9, an alarmin family member secreted after neutrophil and platelet activation that binds to leukocyte TLR-4 and platelet CD36[22] and also triggers Mac-1-dependent NETosis,[23] thereby amplifying both inflammation and arterial and venous thrombosis. S100A8/A9 levels are markedly elevated in the blood of patients with COVID-19.[24]

Finally, platelet-leukocyte interactions are an emerging and active area of research in cancer biology. Platelets and leukocytes interact closely in the bloodstream and within the tumor microenvironment, and their crosstalk contributes to cancer progression by creating a pro-inflammatory and pro-angiogenic niche that nurtures tumor cells and promotes tumor growth, by immune cell evasion through the release of immunosuppressive cytokines, by promoting tumor cell invasion and metastasis, and by promoting thrombosis, which provides a scaffold for metastasis. The findings and experimental methods of Peshkova et al[17] will be very helpful in future clinical trials targeting PLAs in multiple areas of unmet clinical need.



Publication History

Received: 30 December 2025

Accepted: 08 January 2026

Accepted Manuscript online:
12 January 2026

Article published online:
21 January 2026

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